1. Field of the invention
The present invention relates to a mechanism for adjusting the focal position error of exposure heads in an image forming device with a wide exposure width by disposing plural exposure heads.
2. Description of the Related Art
Image forming devices which handle wide paper sizes such as A0 sized paper conventionally use a single A0 size LED head, but these LED heads have poor production yields and low demand quantities, so the cost is high and the LED head will be high priced.
Therefore, in recent years, technology to expose a wide region using multiple heads of a small common size has been used in order to reduce costs.
For instance, as shown in FIG. 9A, exposure device R100 is made with heads h101, h102, h103 fixed in a housing k100. As shown in FIG. 9B which is a view in the direction of arrow A in FIG. 9A, the light emitted from the LED contained in each of the heads h101, h102, h103 passes through lenses r101, r102, r103 respectively which are located at the external boundary regions, and the light is collected and exposed with connecting focal points on the outer circumferential surface of a cylindrical photosensitive drum kd.
In order to obtain an A0 size exposure width, the exposure device R100 aligns three A3 size heads h101, h102, h103 and longitudinally connects the exposure regions of the lenses r101, r102, r103 of each head to ensure the desired exposure width.
At this time, image shift occurring at the seams of the exposure regions for each head h101, h102, h103 will be a problem, and technology to reduce this effect, such as to randomly switch the image information from one head to another head for each scan line in order to make the image shift less noticeable, has been introduced in Japanese Patent Application Laid-open No. 6-255175.
Furthermore, the Japanese Patent Application Laid-open No. 2002-52727 shows a method for detecting and correcting image shift, as well as a method for correcting intensity variation at the head exposure region seams.
Incidentally, the method for determining the focal position of the LED head for focal position adjusting of an exposure device generally uses a method of establishing a focal direction striking member on both sides of the head and adjusting the position using this member. Technology where the striking member which contacts and rotates the photosensitive drum is a space roller is shown in Japanese Patent Application Laid-open No. 62-175782.
Furthermore, as a means for space roller application, Japanese Patent Application Laid-open No. 5-127465 introduces a technology where plural heads are connected and arranged on the inside of a transparent photosensitive drum, and each head is positioned on the drum inner circumferential surface by space rollers. This technology has an advantage in that wearing of the outer circumferential surface of the photoreceptor does not occur, but currently transparent photosensitive bodies are not common.
Therefore, as shown in FIG. 9A, when space rollers are used for a exposure device R100 which has a housing with plural heads mounted in a staggered pattern, a method is conceivable wherein image formation dot lines kl1, kl2 are divided into two columns, so rollers c are established in 4 locations as shown in FIG. 9A, and striking is performed on four points on the outer circumferential surface of the photosensitive drum kd.
On the other hand, when space rollers are not used for mounting of the exposure devices R100, a three-point support system which is the basis for component positioning is generally used for mounting of the exposure devices R100 to the image forming device. Of these three points, two points are for focal positioning and receive the focal direction striking member of both sides of the LED head, and the third point is for controlling the angle of the head to point the exposure direction toward the center of the drum. This technology is shown in Japanese Patent Application Laid-open No. 2002-14524, and this technology has been commonly used in the past.
Note, there are also cases where one of the three-points is able to be adjusted in order to perform fine adjustments to the parallelism of the exposure line with regards to the rotational axis of the exposure drum in order to prevent color shifting of the heads in a color printer which uses plural heads.
Incidentally, when adjusting the focal position of the exposure device, as shown in Japanese Patent Application Laid-open No. 62-175782, when plural heads are joined together and used, the position on both sides of each of the heads will not necessarily be outside of the image area, and if the space roller is made to roll and contact inside the image area of the photosensitive drum, wearing of the photosensitive drum will be promoted, and image lines will occur.
On the other hand, if rollers c are established in four locations as shown in FIG. 9A, the method for striking the four points on the outer surface of the photosensitive drum kd is to simultaneously contact all for points on the drum, so either the orientation of the exposure device R100 with regard to the axis of the drum will be made to vary, or the exposure devices R100 must be forcefully pressed onto the drum and the housing k100 must be deformed.
However, in this case, structural instability is possible for positioning with regards to the drum kd because the drum kd itself is the basis and is rotationally driven.
Furthermore, if plural LED heads are mounted on a common housing to make a wide exposure device, an exposure device R100, which for instance has three LED heads h101, h102, h103 in a staggered arrangement on a housing k100 as shown in FIG. 9A, has focal positions 101a, 101b, 102a, 102b, 103a, 103b to be optimized on both sides of each of the heads h101, h102, h103, or in other words at six locations.
With this structure, if there is error in the position of these six locations during the assembly of the exposure device R100, optimizing all of the focal positions for the LED heads h101, h102, h103 by adjusting the mounting of the exposure device R100 will not be possible. Furthermore, there is an individual difference in the position error on the photosensitive drum kd side for each device.
Therefore, in order to completely eliminate differences in intensity which occurs at the seam lines for the heads, or in other words at the edge region 101b for head h101, edge region 102b for head h102, and edge region 103b for head h103, the focal point of each head must be readjusted while matching the individual differences of the devices.
However, the focal depth of the LED heads is only several tens of micrometers and great skill is required for such minute adjustments, and therefore there is a problem with long adjustment times.